I. Field of the Invention
This invention relates generally to cardiac rhythm management devices, and more particularly to a method and apparatus for avoiding undersensing of episodes of tachycardia due to brady/tachy crossover in the device.
II. Discussion of the Prior Art
Many rate enhancements found in state-of-art multi-chamber pacemakers and pacemaker/defibrillators may cause the device to pace the heart at rates in the excess of a programmed Lower Rate Limit (LRL). For example, features now commonly found in CRMDs, such as rate smoothing, rate adaption, ventricular rate regulation, etc., may cause the device to generate pace pulses faster than the programmed LRL and as fast as a programmed Upper Rate Limit (URL). For devices that detect and/or treat tachyarrhythmias, it often happens that the heart rate due to the tachycardia episode and the programmed URL share a common range of heart rate. That is, at rates near or at the URL, the atrial pace can be inside the lowest tachy zone (LTZ) as a condition referred to as “brady/tachy cross-over”.
For situations of brady/tachy crossover, if the CRMD is pacing the heart at a fast rate when a ventricular tachyarrhythmia occurs, then detection and possible treatment of the tachyarrhythmia is often delayed or sometimes completely inhibited due to undersensing of the tachyarrhythmia caused by ventricular sense channel refractory periods initiated by atrial pacing.
In accordance with the present invention, a pacing mode switch algorithm is implemented in the software/firmware of the microprocessor-based controller used in implementing the CRMD that addresses incidences of brady/tachy crossover whereby undersensing of the tachyarrhythmia due to ventricular sensed channel refractory periods initiated by atrial pacing is avoided. The algorithm operates such that whenever the device is scheduled to deliver an atrial pace pulse at an interval relative to the previous ventricular sense or pace that is shorter than the longest known interval of the tachyarrhythmia, the device delays generation of the atrial pace pulse. The next scheduled ventricular pace pulse would still be delivered so long as it is not inhibited by a ventricular sensed event.
The mode switch is only applied on those cardiac cycles where the V-to-A pace interval is shorter than a programmed ventricular tachycardia interval referred to as the “lowest tachy zone”. The algorithm of the present invention also helps reduce undersensing of atrial arrhythmias due to atrial sensed channel refractory periods that are initiated by atrial pacing, although it will not eliminate that phenomenon entirely.